The present invention relates to a process for preparing hydrolyzed ethylene-vinyl acetate copolymers having improved heat stability.
Hydrolyzed ethylene-vinyl acetate copolymers essentially possess advantageous characteristics such as antistatic property, rigidness, abrasion-resistance, resistances against oils and solvents, little shrinkage in molding procedure, mechanical strength, surface gloss when molded, and high resistance to oxygen permeability and, therefore, it is expected for use as engineering plastics and wrapping or packaging materials.
In general, the hydrolyzed ethylene-vinyl acetate copolymers are prepared by hydrolyzing an ethylene-vinyl acetate copolymer in a medium in the presence of an alkaline catalyst such as hydroxides of alkali metals or methoxides of alkali metals, neutralizing a catalyst by means of an acid such as acetic acid if required, and recovering the hydrolyzed copolymer. However, when the hydrolyzed copolymer so prepared is melt-molded by a molding machine such as an injection molding machine, it is very difficult to continue the molding for a long period of time because the heat stability of the hydrolyzed copolymer is not necessarily sufficient, and also the discoloring and foaming are observed on the obtained molded article. Further, when the hydrolyzed copolymer is re-molded after once molded, the foaming resulting from generation of gases by thermal decomposition of the copolymer and noticeable discoloring are inevitable. In order to improve the heat stability, it has been tried to incorporate various additives into the hydrolyzed copolymer, but no satisfactory additive or means has been found.